Continuing improvements in the performance of modern aircraft engines have resulted in pressure increases therein, for example, in the high-pressure compressor, of 2 MPa at temperatures up to 650.degree. C. At these extreme conditions, abradable nickel and carbon-base linings of stators, which blade tips of rotors abrade, are only of limited value as the surfaces of such conventional abradable linings are substantially entirely eroded by erosive attack. If the abradable linings are made of materials of greater hardness and erosion resistance, however, the rotor blades are subjected to possible damage and fracture. If the blade tips are hardened by means of complex bonded coatings, such as disclosed in U.S. Pat. No. 4,169,020, wherein abrasive particles are embedded in a metal matrix, this problem can be solved. The abrasive particles are made of metal compounds and are referred to hereafter as hard-metal particles.
In the method disclosed in U.S. Pat. No. 4,169,020, hard-metal particles suspended in an elecrodeposition bath are entrapped during electrodeposition of the matrix metal and fixed on the surface of the part by the electrolytically deposited matrix. This method has a disadvantage that the disposition of the particles in the matrix is uncontrolled, and particles can be introduced into the matrix metal only if they are relatively small and suspendable therein. Ultimately, the composition of the matrix with regard to the base metal cannot be optimized, for the reason that there are limits on the complex matrix compositions that can undergo electrodeposition.
Disclosed in German application Ser. No. P 42 41 420.2 is a method for first fixing the hard-metal particles on the part or substrate using a fused salt bath and then electrodepositing the metal matrix.
This method has the disadvantage that extremely critical process parameters must be maintained during electrodeposition to minimize the loss of hard-metal particles fixed by the salt, considering that the salt dissolves in the electrodeposition bath and the hard-metal particles must simultaneously be secured in position by the metal being deposited. Also, the matrix alloy composition usable in this method is extremely limited and not freely selectable.
Standard brazing or soldering methods cannot be utilized due to the unwettability of the hard-metal particles or by the relatively small contact surfaces between the hard-metal particles and the part or substrate surface, so that only a thin-film bond is achieved, while build-up of a metal matrix largely enveloping the hard-metal particles is not achieved. To fill the interstices with matrix metal, therefore, an additional process operation is needed that often is limited by the unwettability of the hard-metal particles or requires elevated fusion temperatures high enough to dissolve the solder layer.
With state-of-the-art compressors, difficulties are encountered especially when an attempt is made to hardface rotor blades of the last stages of the compressors, considering that the coating area available on the blade tips is relatively small compared to the turbine blades or blade tip surfaces at the earlier stages. Cap-shaped hardfacing on blade tips are disclosed in U.S. Pat. No. 4,169,020 and German application Ser. No. P 41 42 420.2, but these cannot be used on rotor blades in the last compressor stages. Another problem encountered is that the base material increasingly favored for the last compressor stages is titanium. Compared with Fe, Ni or Co-base alloys, titanium-base alloys have a greater tendency to crack. Accordingly, hard-metal particles introduced in layers on the tips of titanium-base alloy rotor blades often act as crack nuclei, or due to the necessary high-temperature heat treatment for the hardfacing operation, the metal structure of the rotor blades is unfavorably affected. Both effects appreciably impair the fatigue strength of the titanium-base alloy rotor blades with conventionally deposited hardfacing. This leads to premature airfoil failures in the last compressor stages and may ultimately render the engine inoperable.